The need for articles produced from nonwoven containing nanofibers has continued to increase. The diameters of nanofibers are generally understood to be less than about 1000 nanometer or one micron. The nanofibers webs are desired due to their high surface area, low pore size, and other characteristics. The nanofibers, also commonly called microfibers or very fine fibers, can be produced by a variety of methods and from a variety of materials. Although several methods have been used, there are drawbacks to each of the methods and producing cost effective nanofibers has been difficult.
Methods of producing nanofibers include a class of methods described by melt fibrillation. Non limiting examples of melt fibrillation methods include melt blowing, melt fiber bursting, and melt film fibrillation. Methods of producing nanofibers, not from melts, are film fibrillation, electro-spinning, and solution spinning. Other methods of producing nanofibers include spinning a larger diameter bi-component fiber in an islands-in-the-sea, segmented pie, or other configuration where the fiber is then further processed so that nanofibers result.
Melt fibrillation is a general class of making fibers defined in that one or more polymers are molten and extruded into many possible configurations (e.g. co-extrusion, homogeneous or bicomponent films or filaments) and then fibrillated or fiberized into filaments.
Melt blowing is a commonly used method of producing fibers. Typical fiber diameters range from 2 to 8 micron. Melt blowing can be used to make fibers with smaller diameters but with considerable changes needed to the process. Commonly, redesigned nozzles and dies are needed. Examples of these include U.S. Pat. Nos. 5,679,379 and 6,114,017 by Fabbricante et al. and U.S. Pat. Nos. 5,260,003 and 5,114,631 by Nyssen et al. These methods utilize relatively high pressures, temperatures, and velocities to achieve the small fiber diameter.
Melt fiber bursting is a derivative of mineral fiber making process that has been applied to polymer fiber making. Examples of mineral melt fiber bursting process include U.S. Pat. No. 4,001,357 by Walz et al. and U.S. Pat. Nos. 4,337,074 and 4,533,376 by Muschelknautz et al. The key to this process is the use of sonic and supersonic air (gas) velocities to burst the melt filament into a multiplicity of fine fibers. Typical fiber diameters range from less than 1 micron to about 6 micron. Examples of processes with bursting polymer melt into fine fibers include U.S. Pat. No. 5,075,161 by Nyssen et al.; European Patent Nos. 1 192 301 B1 and 0 724 029 B1 and European Patent Application 1 358 369 A2 by Gerking; and WO 04/020722 by Sodemann et al. These methods utilize Laval nozzles to speed up the gas velocities to sonic and/or supersonic range. When polymer melt is exposed to such high gas velocities, it bursts into multiplicity of fine fibers. The processes are configured by use of desired process conditions and die and nozzle geometries to produce desired fiber sizes.
Melt film fibrillation is another method to produce fibers. A melt film is produced from the melt and then a fluid is used to form nanofibers from the melt film. Two examples of this method include Torobin's U.S. Pat. Nos. 6,315,806; 5,183,670, and 4,536,361; and Reneker's U.S. Pat. Nos. 6,382,526, 6,520,425 and 6,695,992, assigned to the University of Akron.
Film fibrillation is another method of producing nanofibers although not designed for the production of polymeric nanofibers to be used in nonwoven webs. U.S. Pat. No. 6,110,588 by Perez et al., assigned to 3M, describes of method of imparting fluid energy to a surface of a highly oriented, highly crystalline, melt-processed, solidified polymer film to form nanofibers. The films and fibers are useful for high strength applications such as reinforcement fibers for polymers or cast building materials such as concrete.
Electrospinning is a commonly used method of producing nanofibers. In this method, a polymer is dissolved in a solvent and placed in a chamber sealed at one end with a small opening in a necked down portion at the other end. A high voltage potential is then applied between the polymer solution and a collector near the open end of the chamber. The production rates of this process are very slow and fibers are typically produced in small quantities. Another spinning technique for producing nanofibers is solution or flash spinning which utilizes a solvent.
Two-step methods of producing nanofibers are also known. The first step is to spin a larger diameter multicomponent fiber in an islands-in-the-sea, segmented pie, or other configuration. The larger diameter multicomponent fiber is then split or the sea is dissolved so that nanofibers result in the second step. For example, U.S. Pat. No. 5,290,626 by Nishio et al., assigned to Chisso, and U.S. Pat. No. 5,935,883, by Pike et al., assigned to Kimberly-Clark, describe the islands-in-the-sea and segmented pie methods respectively. These processes involve two sequential steps, making the fibers and dividing the fibers.
To produce disposable articles containing nanofibers that are commercially advantageous, the cost of the nanofibers must be controlled. Equipment, process, process aids, and polymer costs can all be controlled. Therefore, it is an object of the invention to produce nanofibers which are low in cost. It is also desired to form products containing nanofibers for a variety of uses and benefits. The uses include executions such as a diaper, wipe, and absorbent material, among other uses.